In fact, only about a third of the U.S. population is considered suitable donors. And of those, only about 5 percent actually give blood.

But with blood in constant high demand, blood bank officials acknowledge they're scaring off good donors because of how screening tests are processed.

The blood supply must be kept clear of disease, and donated blood must be screened quickly, while it is fresh. The tests that can do this job catch potential killers like HIV and hepatitis — but they also detect problems where there are none.

As a result, some donors get letters from the blood bank and the health department informing them that their blood will not be used. They are told that they've had a "false-positive'' result and that they can try again in eight weeks.

But only about 10 percent ever come back, perhaps unnerved or angered by the process.

"The impact of a false-positive could be devastating," said Dr. German Leparc, chief medical officer of St. Petersburg-based Florida Blood Services.

Leparc estimates that each week, Blood Services sees two to three false-positive results for HIV, three to four for syphilis and about eight for hepatitis.

Among them was Ken Bird, 58, of St. Petersburg, whose false-positive test for syphilis last fall resulted in letters from both the blood bank and the Pinellas Health Department, and questions from his wife of six years.

"She was distraught," Bird said. "Her reaction was distrust."

• • •

Bird, a massage therapist and Christian counselor, gave blood for the first time in September, at a Blood Services mobile unit parked at Bay Point Christian Church in St. Petersburg.

Like all donors, he answered nearly 50 questions about health, travel history, sexual experiences, tattoos or body piercings and drug use. His pulse and blood pressure were measured, and a small sample of blood taken to test hemoglobin. Then a pint of his blood was drawn, along with four vials of blood for testing.

The blood was quarantined while it was run through 14 tests for HIV, hepatitis, syphilis and other conditions.

An initial test for syphilis came back positive, but a second test was negative. Still, as is routine, the blood bank sent Bird a letter, telling him that the results "clearly indicate that you do not have syphilis'' but that his blood still wouldn't be used.

Bacterial or viral infections, advanced age, or autoimmune disorders all could have created the false-positive, but there was no need for Bird to see his doctor, the letter said.

All that was surprising, but not too worrisome, Bird said. "If I had a risky lifestyle, I might have been more concerned.''

What came next was more unsettling: A letter from the health department, hand-delivered, wedged between his front door and the screen door. Bird said neither he nor his wife could understand why a nonproblem was generating not one but two letters, one from a government agency. He still felt sure his health was fine, but he wasn't feeling so good about blood donation.

Leparc estimates that false-positives are a small percentage of the 5,000 blood donations his group collects each week. He figures only 10 percent of people with false-positive results ever come back to donate, but acknowledges that one person's bad experience can turn others off from even trying to give blood.

"Sure, I worry," Leparc said of the impact of false-positive results.

• • •

Under Florida law, laboratories and doctors are required to report to the public health department syphilis test results that are "reactive/positive by any method."

Leparc says there's no need for reporting false-positives, which only generates another upsetting letter, without doing anything to protect public health.

Tests used by blood banks are designed specifically to ensure the safety of the blood supply, not to diagnose disease, he said. That's one reason why blood banks tell potential donors not to use blood donation as a way to get an HIV test.

But Dr. Toye Brewer, a medical epidemiologist for the state Bureau of Sexually Transmitted Disease Prevention, said it's the health department's job to ensure that every positive test for syphilis is followed up. Then why send a letter when a second test comes up false-positive?

Sometimes people are asked to come in for additional confirmatory testing, she said, explaining there are at least three different confirmatory tests for syphilis.

Brewer noted that syphilis rates in Florida are among the highest in the nation, putting the disease "high up on the public health radar."

Leparc says better tests for blood donations — ones that don't cast such a wide net — might also help. He has seen testing improve and expand over his 35 years of practice, and false-positive results have gone down.

AuBuchon said current tests search for antibodies associated with diseases such as HIV, hepatitis and syphilis — but which may not mean the disease itself is present. Newer tests used by blood banks can now look for the agent itself.

But even with those advances, AuBuchon doesn't see blood banks eliminating the use of the highly sensitive, sweeping tests.

"Blood bankers like to wear belts and suspenders," AuBuchon said. "The safety of the blood is that important."

Bird said he spoke with Leparc after learning of his test result, and came away from that conversation with a greater understanding of the testing process.

"The problem is not in (donors like Bird). It's in our test," AuBuchon said. "Usually, they understand. We're doing this for the safety of the blood recipient."

Bird, who has written books about spirituality and healing, says he'll consider becoming one of the few false-positives to go back and donate blood.

"If you're saying you're into wellness in the community, and there's a need for blood in the community, you have to step up," he said.

There is a reason why the air in Tampa Bay is filled with playoff talk. If Thursday night's 12-8 Bucs preseason win over the Jaguars is any indication, it's also going to be filled with footballs thrown by quarterback Jameis Winston.